This website provides access to the research results of the VECTORS project, which can be used to support marine management decisions, policies and governance as well as future research and investment.
VECTORS was a large scale project that brought together more than 200 expert researchers from 16 different countries. It examined the significant changes taking place in European seas, their causes, and the impacts they will have on society.

One of the most influential forms of biological invasions is that of invasive ecosystem engineers (IEE). Such species can have wide-reaching consequences because they alter the physical and chemical structure of ecosystems, changing the rules of existence for broad suites of resident biota. Despite ample emphasis on ecosystem engineering in the invasion literature, research has generally focused on biotic changes, rather than direct effects on ecosystem processes or functions. By providing or modifying habitat, IEE may facilitate native species, but they may also exacerbate abiotic conditions with potentially negative effects on abundance and diversity. Using a global systematic review and meta-analysis we tested the impacts of IEE on the abundance of individual species and communities, on biodiversity and on ecosystem functions in marine and estuarine environments. This systematic review and meta-analysis offers the first quantitative global synthesis of the effects of IEE on both biodiversity and ecosystem functions.

▼

One striking aspect of the studies we assessed was the low number that simultaneously investigated IEE impacts on community/diversity and ecosystem functions in the same system hindering the ability to test the link between the two aspects of ecosystems.

In the past two decades, research on the relationship between biodiversity and ecosystem functions has been at the forefront of ecology, with heated debates around theory, models and empirical evidence linking the two. Studies on this link in the context of ecosystem resilience and stability have also been emerging - although the mechanisms are still poorly understood in many cases. Improved understanding of these relationships has become an urgent need due to the magnitude and speed of human-induced changes to the environment through direct impacts on biota, and thus biodiversity. The need to understand the consequences of that change is critical for management given the close linkage between species, ecosystem functions and services12. This is especially true in the context of bioinvasions, and even more particularly so for invasive engineers that have the potential for very large impacts on both diversity a nd function. In light of this, and the research gap identified here, we encourage future studies to test the effects of IEE on diversity and functions simultaneously (preferably considering multi-functionality) to allow a disentanglement of possible mechanisms of impact in the near future. Because of the clear link between species attributes (traits) and functions, we also highlight the need for the inclusion of species traits analysis in this type of study.

▼

At the individual species level, the overall effect of IEEs was small and negative. No significant overall effect was found in any of the categories where community level effects were assessed, while for most ecosystem functions there was a significant or nearly significant overall effect.

It is important to note that near-zero, overall effect on community parameters may not be a true representation of the real effects of IEE. Conversely, it may point to the complex nature of the interactions between the IEE and the different components of the invaded community. Within a community, some species may be facilitated by the IEE while others may be suppressed, either directly by the invader, or indirectly through competition or predation by species facilitated by the IEE (e.g. through trophic cascades). Overall, these potentially strong effects may thus cancel each other out when measured at the community level (e.g. as seen here on analyses of diversity indices or community biomass). Hence, quantifying effects on the specific components of diversity changing between impacted and non-impacted communities may help to elucidate pathways of impact for IEE. Nonetheless, we expect that it will be challenging to disentangle such complex interactions, especially in field observations and experiments. Opposing effect sizes in individual studies, both in species and community level outcomes, are not surprising, as the strength and direction of the effect of IEE are also presumably highly context-dependent (e.g. the identity of the invasive species, the invaded ecosystem etc.). The subgroup analyses aimed to test if some well-defined categories were likely to influence the strength and direction of the effects that could explain this diversity in responses. However, many sub-groups considered here included a small number of studies, which may have hindered our ability to discriminate those different effects adequately.

▼

A much clearer overall pattern was found for IEE impacts on ecosystem functions. Gross primary productivity was negatively affected by the presence of invasive engineers overall. In contrast, decomposition rates and nutrient fluxes were positively impacted by IEE overall. Growth and metabolic rates generally decreased when invasive engineers were present.

We suggest a conceptual model, which may apply to large invasive primary producers and epibenthic sessile invertebrates (non-burrowing or bioturbators) via four interconnected pathways.

The 3D structure of the invader changes the flow through the system, normally slowing currents near the bottom. Higher deposition of particles result in increased nutrient fluxes due to enhanced transport of solutes and particles from the overlying water column to the sediment, stimulating microbial activity. Slower currents reduce food supply to filter-feeders. These processes lead to reduced primary productivity, mainly of microalgae in the sediments, and the deteriorating conditions negatively affect metabolic functions of many native species.

Many invaders, particularly large or densely packed plants and algae, increase the shading of the invaded environment thereby reducing light penetration, leading to lower primary productivity of small native plants, macroalgae and benthic microalgae. Reduced photosynthesis results in lower food availability for macro- and micro-grazers.

The addition of substrate for settlement and the amelioration of environmental stress by offering more shelter, particularly in areas where natural complexity is low, increases the number of niches available to local (and invasive) species that require specific habitats, but also potentially reduce available habitat for species that are inhabitants of the original low-complexity habitat.

Invasive primary producers and some sessile invertebrates (mussels) create a tight matrix that stabilizes the sediments in soft-bottom habitats. This change in sediment properties facilitates infauna that require more stabilized substrate but excludes species that need loose sediments to be able to move freely.